Gelatinous marine organisms from the Azores

It's very common for swimmers to have their baths in the sea interrupted by the presence of jelly-like organisms. Some of these gelatinous creatures are known as jellyfish (Pelagia noctiluca)Portuguese Man o’ War (Physalia physalis) and salp (Salpa spThese organisms can live in the sea, in the water column or on the surface, and can have different shapes, sizes, and colors. They are very common in the sea and on the beaches of the Azores, especially during spring and summer.

Pelagia noctiluca, known as jellyfish in the Azores, Azoresis one of the most studied jellyfish species. Their umbrellas can reach 3 to 12 cm in diameter and present a variable coloration between mauve, purple, pink, light brown, and yellow. The body is radial and symmetrical and has one single cavity, called gastrovascular cavity. This structure is a digestive cavity with a single opening that is used for ingestion and excretion. They have four long oral arms with wrinkled edges that compose the primary feeding region, and eight long tentacles that start from the margins of their umbrella reaching up to two meters in length.

Since they lack a respiratory, excretory, and circulatory system, they have developed cnids which are cells that have several functions such as prey capture, defense, locomotion, and attachment. The cnids are present in the tentacles as well as in the umbrella (uncommon in jellyfish) and when stimulated secrete toxins. For humans, the sting causes pain but is generally not dangerous and there are no known cases of death.

A peculiar characteristic of these organisms is their bioluminescence (Fig. 1), i.e. they have the ability to emit a low-intensity light that is visible to humans at night. The light is emitted when stimulated by the turbulence created by sea waves or the movement of boats.

This species is opportunistic and has been observed feeding on a wide variety of small organisms, such as planktonic crustaceans (e.g. copepods), mollusc larvae, hydromedusae, fish eggs, and larvae as well as debris suspended in the water and microscopic phytoplankton. The variability of the diet of these organisms suggests that they are generalists and do not have high prey selectivity.

The caravela-portuguesa (Physalia physalis)), often mistaken for a jellyfish, is actually a siphonophore. It is a colonial organism made up of several genetically identical individuals (zooids) specialized in different functions that together allow the colony to operate as a single organism. This colony is composed of 4 zooids and each one is responsible for a specific task such as movement (pneumatophore), reproduction (gonozooids), feeding (gastrozooids), and catching prey (dactylozooids) (Fig. 2).

This species is easily recognized by its sail-shaped gas-filled float, a feature that gave it its name due to its resemblance to 18th century Portuguese warships sailing under sail. This floating structure can have a blue, violet or pink coloration and rise up to 15 cm above the surface of the water. It acts as a floating device and as a sail for the colony allowing it to move with the wind, waves, and sea currents. In dangerous situations on the surface, they can deflate the gas vesicle and submerge briefly.

Submerged in the water and supported by the pneumatophore are their long filamentous tentacles which generally grow up to 10 meters long but can reach 30 meters in length. Each tentacle has small filiform curled structures, called nematocysts, which contain toxic and urticant substances and are responsible for transmitting and injecting venom through contact, paralyzing and killing their prey such as fish.

P. physalis is considered to be the most poisonous of the group of gelatinous organisms found in Portugal. For humans, the sting of these organisms is extremely painful, but rarely lethal. The loggerhead turtle (Caretta caretta) is one of the few predators of the Portuguese snail and is not affected by the venom as it has a very thick skin which prevents the venom from penetrating its body.

The salps are pelagic tunicates belonging to the Salpidae family which are in turn included in the phylum of chordates (animals with a dorsal nerve cord). Despite their similarity to jellyfish, due to their simple body structure and planktonic behavior they are phylogenetically related to vertebrates (animals with a dorsal spine). This species has a cylindrical semi-transparent gelatinous body which can vary in size from a few millimeters to around 10 cm.

It moves by contraction, pumping water through its gelatinous body, being considered one of the most efficient examples of jet propulsion in the animal kingdom. Salps are distributed throughout most of the oceans, occurring in equatorial, subtropical, temperate, and cold waters, and can exhibit a solitary or colony form (Fig. 3). The solitary form, known as oozoites, are small barrel-shaped animals that reproduce asexually. The colonies, known as blastozooids, are composed of aggregated individuals that remain interconnected forming long linear chains.

They are non-selective filter feeders, feeding on a wide variety of particles of different sizes from bacteria to larvae, being phytoplankton their main food. Unlike the species mentioned above, salps are not harmful to humans.

Curiosities

In the Azores, signs have been set up on all monitored beaches through specific flags - yellow (jellyfish alert; low numbers) or red (jellyfish danger; high numbers) to alert swimmers for the presence of these organisms and to prevent and minimize contact with them (Fig. 4). It should be noted that even dead individuals of the Portuguese Man o’ War that wash ashore or parts of their tentacles are equally dangerous and their venom can remain potent for hours or even days after the death of the organism or the separation of the tentacle.

Although sometimes dangerous they are unique and fascinating creatures that make our seas richer in diversity, therefore it is common sense not to interfere, damage or remove these animals from their natural environment. Join us on our whale watching trips and discover more of the mysteries that the Azorean sea has to offer.

Blog article written by our biologist Inês Dias.

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Referências / References

Britannica, T. Editors of Encyclopaedia. Salp. (2023). Encyclopedia Britannica. Disponível em: https://www.britannica.com/animal/salp

Instituto Português do Mar e da Atmosfera (IPMA). GelAvista – Espécies. (2021). Disponível em: https://gelavista.ipma.pt/especies/

Mariottini. G.L., Giacco. E. & Pane. L. The mauve stinger Pelagia noctiluca (Forsskål, 1775). Distribution, ecology, toxicity and epidemiology of stings. A review. (2008). Marine Drugs, 6(3):496-513. doi: 10.3390/md20080025.

Melissa Murray. Salps. The Australian Museum. 2019. Disponível em: https://australian.museum/learn/animals/sea-squirts/what-is-a-salp/

Munro. C., Vue, Z., Behringer, RR. & Dunn, CW. Morphology and development of the Portuguese man of war, Physalia physalis (2019). Scientific Report, 9(1):15522. doi: 10.1038/s41598-019-51842-1.

NOAA. What is a Portuguese Man o’ War? National Ocean Service website. (2023). Disponível em: https://oceanservice.noaa.gov/facts/portuguese-man-o-war.html

Salp. American Oceans (2024). Disponível em: https://www.americanoceans.org/species/what-are-salps/